title: Fine structure of the urnulae of Balaustium mites (Actinotrichida: Erythraeidae) representing peculiar defense organs.
abstract: The urnulae, until now the enigmatic paired dorsal protrusions on idiosoma dorsum in active postlarval forms of Balaustium mites, were studied using electron microscopy. They consist of walls made of unmodified integument, which form a cylinder covered by a roof of thin cuticle. At the posterior border of the urnula, the roof has a crescent slit. On its inner surface, a rather large muscle inserts with several tendons. The roof forms a flap under which the modified columnar epidermal cells containing numerous lipid inclusions are located. These lipids are probably secreted through pore canals of the overlying cuticle. Materials mainly originating from an extensive vesicular tissue situated underneath the columnar cells of the urnula and under the adjacent unmodified epidermis are extruded through the mentioned slit. Our results support previous studies that have suggested a function of the urnulae as defensive organs. Our study further suggests that the agent that provides the repellent effect comes mainly from the vesicular tissue, whereas the columnar cells with their lipid secretions are likely to restore the external secretion layer of the epicuticle after its destruction during the repellent release. Further structural and functional details are discussed and compared with other putative defensive secretory organs.
authors.affiliation: Allgemeine und Systematische Zoologie, Zoologisches Institut und Museum, Ernst-Moritz-Arndt-Universität, J.-S.-Bach-Str. 11/12, 17489 Greifswald, Germany. Electronic address: alberti@uni-greifswald.de.
authors.forename: Gerd
authors.initials: G
authors.lastname: Alberti
authors.affiliation: Department of Invertebrate Systematics and Ecology, Institute of Biology, Wrocław University of Environmental and Life Sciences, Kożuchowska 5b, 51-631 Wrocław, Poland. Electronic address: joanna.makol@up.wroc.pl.
authors.forename: Joanna
authors.initials: J
authors.lastname: Mąkol
mesh_terms: D000825:Animal Structures / Q000502:physiology / Q000648:ultrastructure; D000818:Animals; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D008925:Mites / Q000502:physiology / Q000648:ultrastructure*
keywords: Cuticle; Defense; Functional morphology; Gland; Reflex bleeding; Ultrastructure; Urnulae